The present invention relates to a speed change control and apparatus of an automatic transmission for motor vehicles.
Automatic transmissions installed in motor vehicles comprise a plurality of frictional engaging elements, such as hydraulic multiple disc clutches and hydraulic brakes. To carry a shift change or gear-changing operation, these clutches and brakes are selectively rendered operative. That is, engagement of one of the elements is released while a different element is engaged.
When upshift from the first speed to the second speed, for instance, is effected in an automatic transmission, the engagement of a clutch for establishing the first speed is released, while a clutch for establishing the second speed is engaged. This thereby carries out changeover of clutch connection such that a change rate (Nt)' of the rotational speed of an input shaft of the automatic transmission, i.e., the rotational speed change rate of a turbine of a torque converter, decreases along a target change rate. In this case, the turbine rotational speed Nt decreases from a first-speed synchronous rotational speed N1 to reach a second-speed synchronous rotational speed N2. This occurs both when a vehicle is in a constant-speed running state, and when it is in an accelerative running state, as shown in FIG. 10.
On the other hand, when downshift from second to first is effected, the engagement of the clutch for establishing the second speed is released and the clutch for establishing the first speed is engaged. This thereby carries out changeover of clutch connection such that a change rate (Nt)' of the rotational speed Nt of the turbine increases along a target change rate. In this case, the turbine rotational speed Nt increases from the second-speed synchronous rotational speed N2 to the first-speed synchronous rotational speed N1 both when the vehicle is in a constant-speed running state and when it is in a deceleratlve running state.
Meanwhile, as shown in FIGS. 10 and 11, the first- and second-speed synchronous rotational speeds N1 and N2 increase when the vehicle is in an accelerative state (FIG. 10), and decrease when the vehicle is in a decelerative state (FIG. 11), although these speeds are maintained substantially at constant when the vehicle runs at a constant speed.
According to the aforementioned conventional speed change method, however, when the clutch connection is changed over, the changeover is carried out in such a manner that the change rate (Nt)' of the turbine rotational speed Nt becomes equal to a predetermined target change rate. As a result, a time period (speed change time) required for the turbine rotational speed Nt to change from one at the start of speed change, to an associated one of the synchronous rotational speed changes in dependence of a vehicle running state. (The speed change time decreases by .DELTA. Tu and .DELTA. Td in FIGS. 10 and 11, respectively, as compared with a case wherein the vehicle runs at constant speed).
Thus, in the case of upshift during the accelerative running state of the vehicle, for instance, the synchronous rotational speed N2 increases toward the decreasing turbine rotational speed Nt. As a result, the speed change time becomes short, so that the turbine rotational speed Nt is suddenly synchronized with the synchronous rotational speed N2. This causes a problem such that a great speed change shock occurs. In the case of downshift during the decelerative running state, wherein the synchronous rotational speed N1 decreases toward the increasing turbine rotational speed Nt, the speed change time also becomes short, as in the case of the aforesaid upshift. This causes the turbine rotational speed Nt to be suddenly synchronized with the synchronous rotational speed N1. Thus, the problem of a great speed change shock is presented.